Oil viscosity - why the 20W-50 BS?

Well I had a little window of no rain and dry roads so out came the fish yesterday with the 10W-30 Brad Penn oil. Starts better, cold start "idle" better behaved, cold "idle" pressure (1300-1500) around 70 PSI. When the engine got hot @200* the 650 RPM idle pressure was 20 and 1500 is 40 with pressure rapidly going up to 60+ with RPM. I do think the water is running a tad higher with the thinner oil and not sure why that would be the case...unless one thickness has more synthetic. I believe that syn oil does not transfer heat as well (why it can be used at higher temps) and that may be why. Also coulda been the water level in the rad as it was a tad low - putting in more dropped it by about 20*.

These cars are so fun on the back country roads! Need to quiet mine down a little so it isn't cop bait.